OPENTHREAD_CONFIG_MAC_RETX_DELAY_MIN_BACKOFF_EXPONENT macro

@def OPENTHREAD_CONFIG_MAC_RETX_DELAY_MIN_BACKOFF_EXPONENT Specifies the minimum backoff exponent to start with when adding random delay in between frame transmission retries on no-ack error. It is applicable only when `OPENTHREAD_CONFIG_MAC_ADD_DELAY_ON_NO_ACK_ERROR_BEFORE_RETRY` is enabled.